This invention relates to the removal of hydrogen, a main impurity in a secondary cooling system of a liquid sodium cooled fast breeder reactor, from the system through a cover gas phase, to thereby prolong the life of a secondary cold trap.
The secondary cooling system of a fast breeder reactor is provided therein with a steam generator, in which heat exchange is carried out between a liquid sodium coolant and water via a wall of a heat-transfer tube to generate steam. It is known from past experience in operating a fast breeder that an extremely small part of the steam thus generated in the steam generator is decomposed, and that hydrogen, a product of the decomposition of the steam is, diffused, to passes through the wall of the heat-transfer tube in the steam generator, and enters the sodium in the secondary system. The hydrogen, thus diffused to and entered into the sodium in the secondary system, accounts for the greater part of the impurities therein. Consequently, the greater part of the impurities collected in a secondary cold trap, a refiner for sodium in the secondary system, consists of hydrogen. According to the results of rough calculations of a diffusion rate of hydrogen on the basis of the data collected on the experiments conducted heretofore, it can be predicted that, when several years have passed after starting an operation of a fast breeder reactor, the secondary cold trap is plugged with hydride, and its sodium-refining capability is lost.
According to conventional techniques, the cold trap thus plugged with the collected impurities, is replaced by a new cold trap. Although the replacement of such an impurity-plugged cold trap permits recovering the sodium-refining capability of the cold trap system, it is accomplished by the difficult work of cutting the secondary sodium pipes. In addition, with the existing state of knowledge of cold traps, it is almost impossible to accurately predict the life of the cold trap. Hence, in view of the possibility that the cold trap, in use, is plugged sooner or later, it is necessary to manufacture a spare cold trap in advance.
Thermally regenerating an impurity-plugged cold trap, instead of replacing the same, has been proposed in recent years. According to this method, an impurity-plugged cold trap is made vacuous, the cold trap is heated to increase the temperature thereof to thereby thermally decompose the collected hydrogen impurities therein, and the decomposed hydrogen impurities are vacuum-discharged out of the system. This method does not accompany an operation cutting of the sodium pipes, which is required in the above-described replacement of an impurity-plugged cold trap. Accordingly this thermally regenerating method can be practiced easily, and permits achieving a predetermined object is a comparatively short period of time. However, it takes at least several weeks to completely regenerate a cold trap by this thermal method. In view of the corrosion of a filler material in the trap with sodium, this thermal method cannot be practiced frequently.